Sensors detect physical input and in some instances convert the physical input to electrical or optical output. A device may use the electrical output in a variety of ways. Applications of sensors are widespread and sensors are used in everyday devices, such as mobile devices. Specifically, sensors are gaining popularity for biometric readings, such as for reading fingerprints and for authenticating users. Traditionally, ultrasonic systems have operated by interpreting the echoes from sound waves reflecting off of distant objects. For instance, ultrasonic sensors may generate high-frequency sound waves and evaluate the echo that is received back by the sensor. Ultrasonic sensors generally calculate the time interval between sending the signal and receiving the echo to determine the distance to an incident surface of the distant object. In some implementations such as mobile phones, tablet computers, wearable health-monitoring devices and other mobile devices, the total thickness for the ultrasonic sensor must be small, on the order of one millimeter thick or less, limiting the use of traditional approaches.